1. Field of the Invention
Stable and persistent foams have been found which are based on water soluble polymers containing a small amount of oil soluble or hydrophobic groups. Copolymers of acrylamide and a water insoluble N-alkylacrylamide and terpolymers of acrylamide, a monovalent salt of acrylic acid and a water insoluble N-alkylacrylamide when added to water, which may contain electrolytes, and blended with a gas, such as air, nitrogen or carbon dioxide, forms very stable foams at extremely low polymer concentrations. Surprisingly, these polymers do not reduce surface tension like conventional surfactants and do not require any surfactant to form very tenacious and persistent foams. In contrast with conventional surfactants, foaming tendencies increase with increasing temperature. Thus, foams prepared from the novel polymeric foaming agents of this invention are distinguished by their high temperature stability and long term persistence.
2. Description of the Prior Art
A foam is a dispersion of a gas in a liquid with an extremely high dispersed phase volume, such that the system can essentially be regarded as a network of interconnected liquid films (J. J. Bikerman, Ind. Chem., 57, 56 [1965]). Foams find a variety of useful applications, including fire fighting (J. M. Perri in "Foams: Theory and Industrial Applications", J. J. Bikerman, Ed., Reinhold Publishing Corp., NY, 1953, Chapter 12), foam fracturing of petroleum reservoirs to improve oil recovery (M. W. Conway and L. R. Norman, U. S. Pat. No. 4,453,596), soil cleanup or detergency (J. J. Bikerman, "Foams"; Springer-Verlag Publishers, NY, 1973, page 254) froth flotation in minerals processing (R. B. Booth in "Foams: Theory and Industrial Applications", J. J. Bikerman, Ed., Reinhold Publishing Corp., NY, 1953, Chapter 13) and mobility control in miscible (D. C. Bond and O. C. Holbrook, U.S. Pat. No. 2,866,507) and thermal oil recovery (W. E. Brigham, O. P. Malito and S. K. Sanyal, "A Field Experiement of Steam Drive with In-Situ Foaming", DOE Report No. DOW/SF/11445-2 [1984]).
Foams are usually made by the use of surface active agents which concentrate at gas-liquid interfaces and reduce surface tension. The selection of surfactants and conditions for optimum formation and stabilization of foams is one mainly of art. Classes of surfactants which enable aqueous foams to form include alkyl sulfates (K. G. A. Pankhurst, Trans. Faraday Soc., 37, 496 [1941]) (such as sodium dodecylsulfate), alkyl ether sulfates (E. Gotte, 3rd World Congress Surface Active Agents, Cologne, 1969, 1, 45 [1962]) (such as Alipal CD-128), alkyl ethoxylates (W. B. Satkowski, S. K. Huang and R. L. Liss in "Nonionic Surfactants", M. J. Schick, Ed., Marcel Dekker Publishers, NY, 1966, Chapter 4) (such as Neodol 25-9), alkyl aryl ethoxylates (C. R. Enyeart in "Nonionic Surfactants", M. J. Schick, Ed., Marcel Dekker Publishers, 1966, Chapter 3) (such as Igepal CO-730). Also, certain classes of proteins (such as serum albumin and .beta.-casein) (D. E. Graham and M. C. Phillips in "Foams", R. J. Akers, Ed., Academic Press, NY 1976, Chapter 15) enable foams to form. Foam begins to form only when surfactants are present in concentrations near or above their critical micelle concentration (CMC) and the surface tension of the liquid phase has been significantly reduced. For example, the surface tension of aqueous solutions must be reduced from 72 dynes/cm to about 35 dynes/cm by addition of at least 0.01 parts Alipal CD-128 (CMC=0.03 weight percent) per 100 parts water, or by addition of at least 0.001 parts Neodol 25-9 (CMC=0.002 weight percent) per 100 parts water before foams can form.
The amount of gas which can be dispersed in the liquid phase to form a foam depends not only on the surfactant concentration and type, but also on the salinity and temperature of the system (J. J. Bikerman, "Foams", Springer-Verlag Publishers, NY, 1973). As either temperature or the salt content of the aqueous phase increase, the amount of gas which can be dispersed in the aqueous phase decreases. For example, at 25.degree. C. the amount of gas which can be dispersed in aqueous solutions containing 0.03 parts Neodol 25-9 per 100 parts water decreases by 50% at the salinity of the aqueous phase increases from 0 to 10 parts NaCl per 100 parts water. As the temperature increases from 25.degree. C. to 49.degree. C., the amount of gas which can be dispersed in deionized water decreases by 22%. Thus, foams prepared with conventional surfactants are sensitive to temperature and salinity conditions which limit their usefulness in many applications. While reduced surface tension enables foam films to form, a separate mechanism is sometimes needed to stabilize these films. Thus, protein gums and cellulosics have been suggested (J. J. Bikerman, "Foams", Springer-Verlag Publishers, NY, 1973, page 240) as additives which enhance the persistence of foams made with surfactants. While these approaches provide stable foams under a limited set of conditions, it is difficult to form stable foams under conditions such as high temperature or high salinity, which reduce the viscosification efficiency of these additives. Relatively high concentrations of these additives are needed to significantly increase foam stability. Furthermore, these two component packages may be costly and difficult to formulate.
Any of several methods can be used to disperse a gas into a properly formulated liquid phase to form a foam (J. J. Bikerman, "Foams: Theory and Industrial Applications", Reinhold Publishers, NY, 1953, Chapter 1). Gas may be bubbled into the liquid phase. Gas and liquid phases can be coinjected through certain types of nozzles or separately injected through opposing nozzles (for example, H. B. Peterson, R. R. Neill and E. J. Jablonski, Ind. Eng. Chem., 48, 2031 [1956]). Gas can be mixed into the liquid phase within high shear devices, such as blenders (for example, as described in ASTM D3519-76, 1982, "Foam In Aqueous Media [Blender Test]"). Foams can be made when gas is dispersed into a properly formulated liquid phase by splashing (such as in the Ross-Miles Test, described in ASTM D1173-53, 1953, "Foaming Properties of Surface Active Agents") or by shaking. Foams can also be produced by reducing the pressure on a liquid phase which initially contains dissolved gas; when the pressure is reduced below the bubble point of the mixture a separate gas phase can begin to form and, thereby, produce a foam. These methods differ in the amount of gas which can be dispersed into the liquid phase; however, for a single method (e.g., ASTM D1173-53) the volume of gas which is dispersed into the liquid phase can be used as a measure of the relative foam formation and foam stability characteristics of different systems.
While polymers have been previously used in conjunction with surfactants to produce stable foams, there appears to be little use made of high molecular weight water soluble polymers for foaming water or brine in the absence of surfactants. A class of polymers, the subject of this invention, have been found to be efficient foam producers and stabilizers at low concentrations and at elevated temperatures. These polymers are disclosed in U.S. Pat. No. 4,520,182 and processes for synthesizing them were described in U.S. Pat. Nos. 4,528,348 and 4,521,580 and in copending applications attorney docket numbers C-1513 and C-1935, which are hereby incorporated by way of reference. They consist predominantly of acrylamide copolymerized with small amounts of a hydrophobic monomer, such as octylacrylamide, and are designated as "RAM" copolymers. When partially hydrolyzed by the use of alkali base to produce copolymers of metal salts of acrylic acid they are designated as "HRAM" copolymers. They have high molecular weights and are also relatively efficient viscosifiers of water and brine.